BEIJING — China’s capital region remained swathed Monday in a cloud of choking smog, prompting a rise in hospital visits and sales of indoor air purifiers and reports of rare industry shutdowns.

China’s Ministry of Environmental Protection on Sunday dispatched inspection teams to fine and shut down polluting industries in the region, and there were reports that regulators had idled a major concrete kiln and other factories outside Beijing.

On Friday, Beijing raised its pollution alert to the second-highest level for the first time, which meant some manufacturing plants had to suspend or reduce production, and that demolition work, barbecues and fireworks were banned.

Trucks were spraying Beijing’s roads, including in the financial district, as part of an increase in road cleaning, and more people than usual were wearing masks.

In Beijing, 36 companies have halted production and another 75 have cut their output since Friday, Xinhua said, citing the Beijing Municipal Commission of Economy and Information Technology.

Shijiazhuang, the capital of Hebei province, on Sunday ordered 20 percent of private vehicles off the roads in urban areas based on license-plate numbers, the official Xinhua news agency reported.

Tianjin, Langfang, Handan and Baoding also limited the number of vehicles on the roads, according to the China News Service. Tianjin prohibited all construction sites from conducting any earth and stone works, according to Beijing Times report Monday.

But the shutdowns did little to end a four-day bout of heavy particulate smog. Nor are they likely to ameliorate skepticism among residents and outside experts about China’s commitment to environmental protection.

Alex Wang, who teaches law at the University of California, Los Angeles, said China had extensive environmental laws on the books and an increasingly sophisticated ability to monitor sources of smog.

“The problem is not a lack of knowledge about pollution sources,” said Wang, who previously headed the Beijing office of the Natural Resources Defense Council. “Rather, the problem is that environmental regulators lack sufficient authority to deter polluters from violating the law.”

Beijing’s 5 million vehicles are an increasing contributor to the city’s air pollution, but the biggest sources are thought to be industries, smelters and utilities outside the city that use coal as a power source.

On Beijing’s worst days, the smell of coal soot hangs heavy in the air. At 6 p.m. Monday, the air monitor at the U.S. Embassy in Beijing reported that levels of so-called PM 2.5 contaminants — fine particles produced by coal burning that pose the worst risk to human health — had topped 400 micrograms per cubic meter. That’s about 16 times higher than the World Health Organization deems safe, and about five times higher than recent soot levels in Los Angeles.

Beijing’s recent smog bout started more than a week ago and intensified Friday, when authorities issued a code orange alert, reserved for heavy smog that lasts for at least three consecutive days. It was the first time authorities had issued such an alert since they established the color-code system — with red reserved for the absolute worst conditions — in October.

While Beijing residents are accustomed to periods of filthy air every winter, the latest pall is testing the patience of many in the city.

“Beijing’s air is so bad,” one Beijing blogger, Ming Hui, wrote on Sina Weibo, China’s version of Twitter. “What are the relevant government departments doing? … Should enterprises and businesses that emit pollutants buy air purifiers for the people?”

According to a report Sunday in the Beijing Morning Post, the number of people going to the respiratory wards of various hospitals in the city has increased 20 to 50 percent since Friday.

On the street Monday, pedestrians outfitted in masks were far more visible than the week before, although a large number — mainly men — wore no protection and could be seen enjoying the outdoors by lighting up cigarettes.

Health-minded Beijingers have good reason to be skeptical about continuing official pledges to protect their lungs. While China has shown that it can clamp down on emissions — it did during the 2008 Olympics in Beijing — the government’s ambitious industrial-growth targets, powered by the world’s largest consumption of coal, trump its environmental goals. And far too often, Wang and other experts say, the government doesn’t want to risk blowback from industries or even citizens by enforcing environmental laws.

During the recent Chinese New Year celebrations, for instance, Beijing warned that it would ban fireworks if the resulting smoke might contribute to already unsafe smog levels. On Feb. 14, the final day of the celebrations, no such ban was ordered, even though conditions were smoggy. That night, Beijing was ablaze with fireworks and smoke. Within a few hours, levels of fine particulates spiked above 400 micrograms per cubic meter.

China’s widely publicized pollution is blamed for a drop in tourism last year, and industries facing shutdowns on smoggy days complain that they can’t remain competitive. Yet the smog appears to be good for one industry: businesses that sell air purifiers. State-run China National Radio, quoting a store employee, reported Monday that sales of the machines had tripled at one Beijing electronics store in the last few days.

More than one Weibo blogger Monday extolled the benefits of using air purifiers, which can cost more than 15,000 yuan — $2,500 — in major Chinese cities.

“Now I’m sitting in front of it and breathing in as much as I can,” wrote Pink Lipstick 2011. “I want to squeeze out the haze I breathed in the lungs yesterday, and put fresh air in.”